1. Field of the Invention
This invention relates to the recovery of oil from subterranean reservoirs, and more particularly to a new and improved thermal recovery process wherein the oil and gas production is alternately throttled at high and low rates.
2. Description of the Prior Art
In the recovery of petroleum crude oils from subterranean reservoirs, it usually is possible to recover only a minor portion of the oil originally in place in a reservoir by the so-called primary recovery methods, i.e., those methods which utilize only the natural forces present in the reservoir. Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. In these supplemental techniques which are commonly referred to as secondary recovery operations, although they may be primary or tertiary in sequence of employment, energy is supplied to the reservoir as a means of moving the oil in the reservoir to suitable production wells through which it may be withdrawn to the surface of the earth. Perhaps the most common secondary recovery processes are those in which displacing fluids such as water or gas are injected into an oil-bearing reservoir in order to displace the oil therein to suitable production wells. Other widely known secondary recovery or production stimulation processes are the so-called "huff and puff" gas injection techniques such as the procedure disclosed by U.S. Pat. No. 3,123,134 to J. R. Kyte et al. In this procedure, the reservoir typically is closed off to production and a suitable gas such as air, natural gas, combustion products, etc., is injected into the reservoir. Thereafter, gas injection is discontinued and the reservoir is placed on production through the wells used for gas injection and/or additional production wells.
Another secondary recovery process which has shown promise is the concurrent or forward burn in-situ combustion technique. In this procedure, a portion of the reservoir oil is burned or oxidized in-situ to create a combustion front. This combustion front is advanced through the reservoir in the direction of one or more production wells by the injection of a combustion-supporting gas through one or more injection wells. The combustion front is preceded by a high temperature zone, commonly called a "retort zone," within which the reservoir oil is heated to effect a viscosity reduction and is subjected to distillation and cracking. Hydrocarbon fluids including the heated, relatively low viscosity oil and the distillation and cracking products of the oil then are displaced toward production wells where they are subsequently withdrawn to the surface of the earth. The in-situ combustion procedure is particularly useful in the recovery of thick, heavy oils such as viscous petroleum crude oils and the heavy, tar-like hydrocarbons present in tar sands. While these tar-like hydrocarbons may exist as solid or semi-solid materials in their native state, they undergo a sharp viscosity reduction upon heating and in the position of the reservoir where the temperature has been increased by the in-situ combustion process behave like the more conventional petroleum crude oils.
In in-situ combustion oil recovery procedures, various techniques have been proposed which involve the manipulation of one or more production wells in the recovery pattern. These techniques typically are for the purpose of controlling the movement of the combustion front or the flow of fluids within the formation, particularly those fluids in the vicinity of the retort zone and combustion zone. Thus, in U.S. Pat. No. 2,390,770 to Barton et al., there is disclosed a procedure for controlling the movement of the combustion front by such procedures as throttling, to the extent if necessary of closing, a production well toward which the combustion front is preferentially moving and/or injecting various fluids such as drilling mud or water into such a well. Also, in U.S. Pat. No. 2,862,557 to van Utenhove et al. there is disclosed an in-situ combustion process in which gas is injected through a production well in order to bring about a pressure gradient reversal within the formation so as to force condensed products away from the production well into other portions of the formation.
A variation on the conventional in-situ combustion process in which the production well or wells are alternately throttled to effect an increase in oil recovery is disclosed in U.S. Pat. No. 3,434,541 to Cook et al.
More recently, an improved thermal method for recovering viscous petroleum has been disclosed in U.S. Pat. No. 4,127,172 to Redford et al. which utilizes the use of pressurization and drawdown cycles with the injection of thermal recovery fluids as a mixture of steam and an oxygen-containing gas. Pressurization of the formation, for example, may be accomplished by employing a higher injection rate than the production rate. Thereafter, drawdown, which is a reduction in formation pressure, may be accomplished by producing at a rate greater than the injection rate. In a later patent, U.S. Pat. No. 4,217,956 to Goss et al., an improvement in U.S. Pat. No. 4,127,172 is described wherein carbon dioxide is injected at the start of the pressurization cycle along with the injection of steam or a mixture of steam and an oxygen-containing gas.